Underwater transducer



Filed Dec. 8. 1948 2 SHEETS--SHEET-Z INVENTOR Fi 6. l2 :JM

Patented Oct. 7, 195.2

NUturrso .STATES PATENT oFFicE UNDERWATER TRANSDU'CER A Frank Massa, Cleveland Heights, ofhio 4Mifplicat'or1l'ec'ernber 8, 1948, 4SerialNem $4,142` i My invention is concerned with transducers which are capable of receiving` .or generating compressional waves in 4a` liquid unedium. A'n example of such a transducer is ajhydrophone which converts underwater sound `vibrations to electrical vibrations or, reciprocally, a structure which is actuated by an electrical signal andge'ncrates a corresponding soundsignal. Two lof the most common'forms of transducertelements which are employed for'ig'enerating 'or receiving sound waves in a liquid medium arepiezo-electric crystalszor magneto-striction elements. When trans-r ducer `elements are designedforoperati'on in the audible frequency range, it is generallylnecessary, in cases WhereV the pressure .sensitive element is responsive to sound Spressure applied along .more than one axis, to shieldoertain surfaces of the transducer element to prevent the undesired actuation of these surfaces 'by the sound pressure Wave. For example, if a 45 X-cut or 45 Y'cut Rochelle salt crystal plate is immersed in a liquid medium and arranged so thatsound pressure `may simultaneously reach over al1 of its surfaces, the output voltage of `the crystal will be practically zero for the frequency region in which the Wavelength of sound in the medium is lar'gecompared with any of the crystal dimensions. This zondi-l tion is Well-known and results `from `the fact that equal pressures 'appliedalong the Itwo right angle piezo-electrically` sensitive axes of Vthe 'crystal generate voltages which are .equal in magnitude and opposite in phase, therefore neutralizing each other. Ii" the sound pressure is. prevented from being applied along one of the two sensitive crystal axes, the output voltage of `the crystal will be proportional to the pressure Vapplied alongthe other sensitive axis. i

It is the primary object of `xnyinvention to iin prove the performance of an underwater transducer especially in the audible frequency range.

Another object of my inventionis to .produce a precision, low-cost assembly which is capable of operation in depths of water up to several hundred feet Without appreciable changeinyop- @rating characteristics.V

StillV another object of my invention isto provide a piezo-electric crystal assembly in `which sound vibrations are prevented from Areaching predetermined surfaces of the crystallstructure for the purpose of improving the performance of the structure. Y, J

Another object of myinvention Vis to provide a very accurately-dimensioned layer of ,a low acoustic impedance material between ,predetermined surfaces of a piezoeelectriclassembly and fo eiaims. (o1. 177-38@ -A -further object 'of my invention is to reduce the resonant frequency vof a piezoeelec'tric element whereby increased sensitivity resultsat the reouirediower `frequency region.

Another object :of my invention is to provide a simple, rugged mounting for my vimproved transducer 'assembly whereby minimum obstruotion is presented to the sound field. i

A further 'obiect fof my invention is .to Aprovide a rugged, lowecost'mean's for assembling a 'number 'of my .improved ,transducer .elements in 'order `to obtain `highly directional characteristics for the structure. i

`lin'otler oloe'ctqof `my inventionY is t assemble an .improved structure in an `eiiicient; low-'cost reflector whereby further control is achieved in the directional characteristi'cof the unit. p,

The novel features that I consider characteristie .of ymy invention `are .set forth with particularity inthe appended claims- `The invention,

itself, hotveveg'both Aas to .its organization andV method of operatiomas'iwell as additional objects and advantages thereof, Will best be understood f'romthe following description of several embodi ments thereofvhentread finconne'ction with the accompanying draivingsn which;

Fig; lis aplan viewof one embodiment of `my inventin. i

Fig.` 2 is' ase'ction of Fig.v 1. l

Fig. 3 is an exploded 'View in perspective showin'gfthe structure of 1in greater'detail.

Fig. 4 isha plan vieu/of yanothenfornoii my in= v'entin.` i i Fig. 6 is a plan view of still another forrri of 'my invention. v

Fie. c isa 4section along the une 1:1 of Fig. 6.

Fig". v'8 is' a 4shei'natic Wiring diagram shovvin the eleetr'ical onnetins as they` are triade in `one type of piezo-electric crystal assembly.-

Fig. 9 is apartial sectional vivv of a hydr'# phone embodying' my improved transducer lnient.

Fifi; l() is 5a partial sectional View'v showing a" une assembiyof several of my-improvdtrans# ducer elements.

Fig. 11 is an end view' showing one typeof te sembly f mytra'nsducer `vvithiri `afi angular re-v e'ctor to secure certain desired direct-ionalYchar-` acteristics. l i E FigJfIZ shows a partial cuteawaylongitudinal ,View of the structure shown in Fig. 11.`

Referring more particularly tothe gures,

taken along the une `1 .2

which the same reference character represents similar elements, and referring more specifically` applied along the longitudinal axis of the crystal assembly, the individual voltages generated'across each -crystal plate are combined in properjphase relationship andappear across the'outputffter'- V Some common -typesof piezo-,t`

minals 8 and 9. y electric crystals vwhich would satisfy the above Y-cut Rochelle salt or Z-cut ammonium dihydrogen phosphate; The. piezo-electric; properties'of` such` `crystal plates are well-.known and will 'notb'e described furtherin this specification.

The details of the crystal .assembly are most clearly shownwini theupper portion of the exploded perspective View of Figu3 taken in conjunction With....the schematic Awiring-:diagram -which'is showr'rirr-Fig.` 8. The output terminal 811s connected with. the commonpotential terminals"` of. the .several crystal plates which. are

. indicatedby theminus sign inFig. '8, 'and output terminal 9 is connected withv the opposite common polarity oi the crystal plates indicated by theplus signs in Fig. 8. The polarity indications inv Fig.. 8f representA theinstantaneous charges 4 developed on 'theside faces of theseveral crystal plates k*when a compressional stressv is applied -along fthe vertical axis of ,the crystalfassembly when itjii's mounted` as shown `in Fig. 2. V.When

the crystal from exposure to thesound field, and

although improving the low-frequency performance of the assembly, possesses the disadvantage that it may be mechanically weak and may not withstand shock or submergence in static pres .kif thediaphragms were missing. Such a unie Y laterally appliednystatic stress would result in Vsericus non-linearity in the operationof `the 45DV X-.cut Rochelle salt crystals.

. arrangement are, for example, 45 X-'cut'on'li y Another method which has been employed for improving-the construction of a crystal hydrophone has been to cover the sides of the crystals with'low acoustic impedance material and, after placing the covered crystals into anopen metallic sleeve, -to `immerse-'the assembly inside an oilfllled container lso thatsound pressure applied to theouter wall of the container would be trans-k mitted directlytoth'e crystal structure mounted within it. For; this `type of construction, the hydrostatic pressure, when the hydrophone is submerged, is applied uniformly along all 'axes of .the 'crystal structure. However, because it was notrealizedthat the low acoustic impedance material'has .to be relatively thin and completely ll the space between the sides of the crystals and the housing, thel performance of.4 such a hydrophone structure was quite variable in the kthe stress-reverseswits direction, the indicated polarity-markings in Fig. 18 will also reverse. Thus. an alternating .voltage Vwill appear across thejoutput leads 8 andy 9 corresponding-to altere natingppressures applied alongthe vertical axis of `the crystal. assemblywhen lit isqarranged as shown in Fig. .2. 'The -leads l0 and :l l `which are shown schematically. in Fig.. 8. and also shown in the top view of Fig. 3 employed for establishing electrical vconnection between' the A.side crystal faces and the output terminals 8 and '9. The connections to the terminal 9 are made by the leads l'i'and the vconnections tothe terminal 8 are made by ythe leads l0. f When'the output terminals 8 and 9 are installed, an insulating film i2 is locatedbetween the output "terminals and the crystal edges as shown in the upper por.- tionofEig;3.- j

Itis well-known that if an assemblyoffcrystal plates,` suchv as are shownv inthe top Yviewof lower audible frequencies in the regionbelow 1000 cycles/second. .For hydrophones that had to operatefsatisfactorily very much below 1000 cycles/second, it was found necessary to employ .diaphragms which sealed oi the side faces of thei'crystal .from the sound field and thereby caused ythe sound pressure to be applied substantiallyalong a'single axis of the crystal assembly. A In order to Vobtain uniform sensitivity in the lower audiblefrequen'cy range extendingdown Fig. 3, employing 45 X-cut or 45 Y-cut Rochelle -f salt or 457 l Z-cut ammonium di-'hydrogen `phosphate is immersed' in ya sound iieldflthev sound pressure which would act simultaneously; ronfall facesvof the-crystals throughOut thelowerffrequency` rangef;-would-=,result in practically :zero

electrical output. In order to avoid the loss `in lowffrequencyy output from such a..crystal"'as sembly, it vhas beenV common practicerrto mount such an assembly inside a housingand :to seal olf thehousing bymeans of diaphragme which issued toFrank Massa on-December'Bl, 1946.

This type of construction shieldsfthelsides! of ing uniform sensitivity at the lower audio frequencies is less than 20% of the assembled crystal dimensioni-measured along a line perpendicular to the low acoustic impedance film, and to secure uniform sensitivity'-v to frequenciesY below 100 cycles/second, -I found it desirable to employ nlm thicknesses' of the order of 10% or less of the total crystaldimension above mentioned.

In orderto achieve my desired objective of providing an accurately-controlled thin film of low acousticlrimpedance material between the side surfaces of vvmy crystal assembly and a rigid housing structure, I found it desirable to use the tour 4of the crystal assembly. A layer of low acoustic `impedancematerial I4- is cemented to the innerfsurfaceof thevshells I3, as illustrated in the centerview of. Fig. 3. A suitable type of 'low acoustic impedance mater'ial 1i-s e mixture "o'f 'granulatedcork and '-rubberwhich lis. Imolded ttogether to formacompressible sheet. 'The lthickness of the material Il! isfmachinedtofafuriif'orm dimension and it "is adjusted y"so that when/the iinal yassembly is made, as indicated in '-Figs. l and 2, the material I4 is slightly compressed so that it very completely fills the yspace between the crystal assembly 'and the inner wall 'of the shells I3. An 'outer rigid'structure I'is provided 'with va hole along iits axis 'within 'which the assembled shells I3'can be forced. Near thefcent'er terial lili are assembled to enclose therside surfaces ofthe 'crystal assemblyfand this composite structure is 'then forced :into the axialholepro- `vided in the 'housing I iso 'that `acomplete vtrans- -ducer elementresults, Aas Y:shown inthe views of Figs. .l 'and i2. If this `.transducer #element is;im mersed in a liquid, such as castor uoilfor-silicone iiuid, which is non-injurious tothe. crystalsfand sound Waves ,are vset up within the` liquid, `only `the 'exposed end "faces ofjthe crystals will be activated by thesound pressure lbecause the. side crystal faces are eectively isolated Aby the `clonstruction just described.. `The thinlmof llow acoustic impedance `material .LA servesthe dual function of permitting Athe transmission ofstatic pressure alongallaXes of the l. crystals Awhereas it prevents `alternating sound pressures corresponding to the lower audible frequenciesfromreaching the side `faces of the crystal assembly., 'Since only the uncovered end face'sf'o'f thecrystal assembly remain `effectively exposed to the action of the alternating sound pressure, alternating stresses will only be setup inthe `crystalsalong the vertical axis of Fig. 2" thereby generating electrical signals acrossthe terminals 8 "andf'which correspond to the "frequency andwavesh'ape .of the vsound pressure'waves. 'By ,constructingtthe crystal assembly as described 'above wltha film of low acoustic impedance material completely lling the space between thesidesoftlre'crystals and the surrounding rigid WalL `If'ound*that the sensitivity of 'the transducer 'element Whenused as a hydrophcne was uniformto very Llcv'f`fre quencies down to 30 'cycles A"or less. LIfalso found it 'advantageous to malte t the length of the `Icrystals greater than theirwi'dthdimensions; in which case `the 'resulting 'long axial length of ithe low acoustic impedance lfilm as compared with its thickness helpsto 'achievev thel improved performance atthe verylow 'audi-ble frequencies.

A.l modification ofthe construction shownin Figs. 1,'2, and 3 `isindicated"irrFigs""4anti in which the main `Acrystal assembly "4, t5, .6, u'and *l is cemented between .two cylindrical "rods "'I'5. The composite assembly o'fithe'crystalplatesfand the rods I5 is held betweenitwo 'semie'cirjcula'r shells l i5 which are lin'ed with *a* thin' film of low acoustic impedance material I 4 which( isthesarne as the material 'describedin connection with the previous assembly. With "the crystals `antlfthe end pieces I5 held betweenthe'two'lined'sernicircular shells IS, the entire `structure is 'forced into a cylindrical tube I'1 to complete the*trans ducer assembly. Forthe loweraudiblefrequency range, the addition 'of the rods I5 to `the end i'fa'cesef the crystalassenibly fcauses ino 'ap- -preciabl'e -bhangefin performance of Athe 4"transduofer.v some predetermined higher audible Vfrequency, t-hefnse -of-the` Jcomposite structure `rfe'su'ltsfin-'greatly increased outpu't. )The frequency at `which this increasedoutput'res'ults lies Ain the vicinity oi-lthe ,resonant .frequency of lthe composite 'length Afof the "crystal lplates jplus the 'two attacnedjrods I5`; "The reason fior employing :this type of'construction to achieve'resonance `in 'the audible Lfreauen'cy jrange is that it `is impractical to `obtain crystal gplatesflong enough 'to resonatebythemselvesatffrequenciesmuchjlower thanlikc. By.attachine"rods to the end of the crystals', theresonantffvrequency yo'f '.the combined vvstrlli'ttilre maylbe set ias lowes severaljhundred cy'1es`,`i desired.l f i Y Ags'till u r'i'lher `type ,of .fccm-structionv employing myimproved method of .mounting 'the crystals within a? housing is .indicated .in lligs. 1,61 @and .7. :ln .these figures, the sainelgroupof V,crystals A, .5, `I5, and .1 -as were used-in Figs. ,1.and2 are mounted Within the saine two shells ll-which vare .lined 'with the ,low v,acoustic `,impedance `material fifi. Theouter Ahousing I8 in .thisscase :hasonly one o pen end, and when `the crystal sand sshellassem- .bly fis forcedinto the openingsof -thefhousing le, l.only-one jend face :of the crystal assembly fre-- Nmains exposed for actuation :in the fsound field. .As;shown .in Fig. 7, -a vdisc of .low facoustic irn- `pedance i material `I 4' iis shown :intermediate be tween the insidewbottom surface of the ihousine I8 and one `of 'the end .faces Lof rthecrystal :assemfbly. The iessentialtdilerencetbetween:the con- Astruction :of Figs. :6` and .7 as toomparedpwith the `construction.described @the :previous fgures is that .the directional y,'charaeteristic/ of `the construction of Figs. 6 and 7 will .be essentially ithatiof atplan'e .piston havingcthercontour shown fby the 'outer periphery'of the@crystabassembly,V as lindicatedlinithelplan view Lof FigJB. :In theipre- :vious r structures, :both ends fof i the crystal assemfbl-y "are lexposed `to .actuation by fthe sound field rand, therefore, the `directional a-patterns .in these arrangements iwill 4be 'equivalent `to that corre- "spondingto a :pair of piston sources .l displaced laxially corresponding vto fthe total length lof .the transducer element assembly.

Fig. `9 isla partial sectional viewlindica'ting-one method which AI have successfully employed :for mounting the transducer element abovel described lto1make la satisfactory operating '.hydrophone in whichthe mounting structure-oliers minimum obstruction to the soundiield.. "lhefhousing `I vwhich contains the active portion fofvthe transducer f element is `mountedvbetween two .UA-shaped Wire rods I9 vwhich are rigidly secured to the diange portion 2D. kflour drive screws fdl are employed for `-fastening 'the "housingrfi i securely 1to the wire frame, as lindicated. The Lwire iframe r"has flattened portions in the region `where fthe drive "screws fare-employed. The 'crystal outp-ut-terrnnais '8 andV Blare soldered tol thefccnductors 2v2 and they other-ends of the Aconducto-rs jar-e :soldered to the*insulated-terminals Vd'3 Awliiclif-"are, in turn, `sealf-:d to -the housing Lll.` 'illoops of wire "25 are 'employedlfor' holding'f the conductors Z2 Asecurely againstA the 'wire frame.' as indicated. The transducer assemblyfcomprising the fiangeportion 20, tcgether'with the wire frame structure and the transducer element,` is inserted Ainto 'the Aouter housing structure -through"the openingin the cylindrical tube "26. `)The tube 2"6 is .'soldered to asecondportion 21 "and arubber bootje is bondedi-"to tlreportion 21, 'a'sindicated in the view.

When the transducer assembly isinserted through the opening in the tube 26, thelouter ring of the flange portion will sit on the shoulder provided in part 21'. A sealinggasket 29 and a clamp ring are then dropped Vinto position, as shown, and a ring nut 3I is screwedinto the threads which are formed'on the inner surface. of part 21, as indicated, to hermetically seal the complete assembly within the space into'wh'ic'h the transducer element is mounted. 'After the ring nut'3l is securely fastened, the space inside the rubber jacket is evacuated through the vpipe tap 32. 'After completely removing the' air' from the innercompartment, a suitable soundconducting 'liquidsuch as castor oil or silicone iiui'd isinj'ected into the space through the opening 32 andjapipe plugis then inserted in the opening 32to `seal theliquid within the chamber. In order to completely fill the housing with the liquid, itis preferable to employ a rigid sleeve surrounding the outer surface of the rubber boot 28 and to create a Vacuum between'the outer wall vof the rubber bootand the external sleeve for the purpose of preventing the collapse of the rubber boot during the -evacuationof its inner volume'. 4'Iv'vo'iiexible leadsy 33 are soldered to the exposed ends of the insulated terminals `23 as indicated and they serve to permit external connection to the transducer terminals 8 and 9 which are enclosed in the liquidlled compartment. The space within the tubular portion 26 may be employed for lmounting a preamplifier for applications in which the transofthe-terminals 2 3,v as shown. A bushing 42 whic ontains the pipe tap 43 is bonded to the rubberboot 281 and is employed as the means for- ;evacuatingand o ilfnlling the transducer struc- -tureinrthesame mannerv as described'in connecf tion witlr evacuationand filling of the assembly .Another f orm of aline type assembly is shown in; Figa-.ill .and l112. 'In this" case, a number of housingstructures II- carryin'g the active asseml,bliesfdes cfribed in Fig; 5 are .mounted between vthe.v-.rc'idsd'I ,lwas previously 'described in Fig. 1Q. Small-.Spacing members ,44;- are fastened to the outer .periphery of v thehousings I 1, as shown, and

ldrive screws; 2I. A layer of low acousticimped- --ance material. I4 covers the rear portion of the -reflector,;;as.shown,. and-serves to attenuate the ducer element is employed as ay hydrophone and in which the preampliiierV is usually necessary. If the preamplifier is not required.' the length of. the cylindrical housing 26 may; beshortenedanda Waterproof sealing .arrangement for 'the cable may be provided similarto that' shown in thetcp View ofFig.10.

` In Fig. 10, is shown an arrangementin l.which a number of transducer elements similar to the element mounted in Fig. -.9 are arranged .to form a long line of elements which are all connected electrically together and `thereby result-in sharp directional characteristics forsoundsarriving in any plane containing the vertical-axis of theline. In Fig, 10, two U-shaped wire structures I9 .are

employed similar tothose shown in Fig. 9ex

cept that in Fig. 10, the wire Us are made longer and provisions are made for receiving several of the housings I, as indicated. The housings .-Lare secured to the wire rods by means of the dr i ve screws 2I, as previously described in connection with Fig. 9. The crystal terminals 8 and 9 ex,- tend through both the top and bottomportion of most of the housings I in order to permit the parallel connection of I all the terminals by means of the Wires 22, as shown.vv Wire clamps A25 are employed for securing the conductorsV 22 to the U -shaped rods I9 in the same manner as described in connection with Fig. 9. The wire rods liare secured to the flange portion 34 and the electrical connections from the transducerV elements are made through theinsulated terminals 23 which seal off the enclosure containing the transducer elements from the outside region. A rubber sleeve 28 is sealed to the surface of portion 34.afterthe transducer elements are assembled in.place. 1`'he f inside surface of the portion 34 is machinedto receive a washer 35, a packing gland 36, and-a second washer 31, ais-indicated, and anut` 38 .Whichis screwed into the portionv 34 willserve to compress the packing. gland 36 to etico@ .a water-tight seal between the cable 39 andthe 'inner Vwall of the portion 34.- The two cablecondu tors .40 .and .4I areconnected to the exposed.A ends sound'which would 'otherwise pass through the rear ofthe reflector plate were the low acoustic impedancematerial I4 omitted. For the transduc'er.' construction indicated in Figs. 11 and 12, theeifective -sound pickup angle in a plane nor-v 'mal'to thel axis of the line will be essentially as defined bythe-angular openingof the reflector 'plate 45 for frequencies in which the Wavelength of "sour idis large when compared with the dimensin ofthe opening across the reflector.

'Althoughl have vchosen only afew specific ex- 'amplesjtfo 'illustrate the basic principles of my in ventionlfitwill'be obvious to those skilled in fthe; art th' [numerous departures may be made lfrom thendet'ailsvshown and I, therefore, desire that my"'nyention 'shall not be limited except insofarjasismade necessary by the prior art and .by the spirit :ofthe appended claims.

I claim' as my invention:

` 1'. In'v an underwater transducer capable of generating'o'r receiving sound waves in the lower audible'., frequency range, an assembly lof one or more piezo-electric crystal elements, each element'havinga pair of end faces vand a number of'side faces.each element characterized in that an electricalvoltage Will be generated within the element-,by the application of an alternating mef chanical `pressure along an axis normal to said pairof endfaces and-also along an axisat right angles Eto said normal-axis, a rigid Walled housing Lstructure,havingat least one open end surrounding said crystal assembly, theinner wall of said housing structure beingclosely spaced from at leastone l-oi ,the side vfaces of said crystal Iassembly.' at least one of the end faces of said crystal assembly being vexposed through the open end of, said `housing'fandalayer of material of lower acoustic impedancethan the crystal substance substantially lling the space between the inner wall of said housing'and the side face of the crystal': element mounted within said housing, the thicknessv of VVsaid low acoustic impedance material being of the order of 10% or less of the crystal element dimension measured in a direction at right angles tothe surface of said low acoustic impedance material.

i 2. In'anunderwater transducer capable of generatingv or receiving sound waves in theaudible .frequency rangafthe 'combination of a. piezo-electric crystal element having a pair of end faces and a number of side faces, a number of rigid shell-like sections having an outer contour and an inner contour, the inner contour of said shelllike sections conforming to the outer contour of a portion of the periphery formed by the side faces of said crystal element, a layer of `material lining the inner contour oi the said shell-like sections, said material having a lower acoustic impedance than the acoustic impedance of the crystal substance, said lined shell-like sections arranged to surround the side faces of said crystal element whereby said low acoustic impedance layer is held in close contact with the side faces of said crystal element and a housing structure having an opening within which said assembly of lined shell-like sections together with the enclosed crystal element is rmly held.

3. The invention described in claim v2 characment measured in a direction at right angles to the surface of said low acoustic impedance material, and further characterized in that said low acoustic impedance material is compressed during assembly whereby the entire space between the inner contour of the said shell-like sections and the side faces of said crystal element is completely filled.

5. The invention described in claim 2 characterized in that the thickness of said layer of low acoustic impedance material is of the order ofv 10% or less of the dimension of the crystal element measured in a direction at right angles to the surface of said low acoustic impedance material, and further characterized in that the length of said crystal element is greater than the width of said crystal element measured along any direction perpendicular to the surface of said low acoustic material which surrounds the side faces of said crystal element.

6. In a transducer capable of generating or receiving sound waves in a liquid in the lower audible-frequency range. a transducer element capable of converting alternating pressures to alternating voltages, a housing structure within which said transducer element is mounted, a frame-like support surrounding said housing structure, said frame-like support comprising an assembly of rod-like members whose Width dimensions are small compared with the Width of said housing structure, means for rigidly attaching said housing structure to said frame-like support. an outer enclosure within which said assembled housing structure and frame-like support is mounted, sealed terminal means for establishing electrical connection to the transducer element Within said outer enclosure, and a sound conducting substance lling the space between said transducer element and the inner surface portion of said surrounding enclosure.

7. The invention set forth in claim 6 characterized in that said transducer element comprises an assembly of severial piezo-electric crystal elements, each mounted Within a separate rigid tubular housing structure.

8. The invention set forth in claim 6 characterized in that said transducer element comprises a line assembly of separate transducer elements, an

angular reiiector plate surrounding said line aS- sembly, and means for securing said line assem-` bly along the vertex of said angular reflector plate.

9. In a transducer capable of generating or receiving sound waves in a liquid in the lower audible-frequency range, the combination of a; housing structure with a normal axis whose difaces closely spaced to at least a pair of surfaces c of said transducer elements, and a layer of material having a lower acoustic impedance than i the substance of the transducer element mounted within the spaces between said Wall-like surfaces and said faces of said transducer elements.

10. In an underwater transducer capable of generating or receiving sound waves in the audible-frequency range, the combination of a piezoelectric crystal element having a pair of end faces and a number of side faces, a number of rigid i shell-like sections havingan outer-contour kand an inner contour, a layer of material lining the inner contour of the said shell-like sections, said material having a lower acoustic impedance than the acoustic impedance of the crystal substance, said lined shell-like sections arranged to surround the side faces of vsaid crystal element and means for holding said shell-like sections :in iixed relationship with said crystal element whereby said low acoustic impedance material is held in close contact With said crystal element.

FRANK MASSA.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,985,251 Hayes Dec. 25, 1939 2,076,330 Wood et al Apr. 6, 1937 2,138,036 Kunze Nov. 29, 1938 2,405,605 Goodale et al Aug. ,13, 1946l 2,413,462 Massa (C) Dec. 31, 1946 2,414,489 Shomer Jan. 21, 1947 2,417,829 Keller Mar. 25, 1947 2,435,548 Rosenthal Feb. 3, 1948 2,440,903 Massa May 4, 1948 2,456,294 Massa Dec. 214, 1948 2,480,535 Alois et al Aug. 30, 1949 

